The present invention relates to improvements in a method of making seamless tubes or pipes, wherein a pipe, i.e. a hollow blank, has been made first e.g. by piercing or by extruding a billet.
Hollow blanks made by such a process are often subsequently drawn or stretch-rolled or otherwise reduced in diameter. However, it has been found that, regardless of such working, the resulting tube or pipe still has a bore which is eccentric to its outer surface. The eccentricity occurs predominantly in pipes made by extrusion, and subsequent cold rolling or drawing will not eliminate the eccentricity, at least not completely.
Pipe extrusion presses as used today operate in that the billet chamber receives a heated billet which is pierced by a mandrel, and the billet is extruded by a ram, through a die and around the mandrel. The mandrel assumes a floating disposition in and with respect to the surrounding extrusion flow. Unfortunately, the mandrel may be displaced relative to the die, e.g. up to 10% and that accounts for the eccentricity in the resulting pipes. Pipes made by piercing exhibit a displacement of center lines by about 5%. Subsequent cold rolling or drawing will eliminate some of the eccentricity, but not all of it.